My 1997 528i's electric fan has yet to turn on under any circumstances, when left at idle approching overheating...no fan..with the a/c on approching overheating...no fan. What is the best way to diagnose the problem. I imagine there are relay's for the engine temp, and I would think a seperate system that would trigger the fan once the a/c comes on. My guess is the fan itself because of the fact neither turns it on. What do you guys think?

If you have access to a multimeter, see if the fan is getting juice at the connector. Take a reading with the engine on, A/C off. Then take a reading with the engine on, A/C on. If there is a difference, then your motor is toast.

If there isn't a difference in voltage, then your relay is toast. THen move on to your relay connections to determine if the relay is getting juice or if the ECU is toast.

C'mon men, standard troubleshooting. If you don't have a multimeter, get one.

Thanks I know how to use a multimeter...anyway back to my question for a change. I assume this is a multi speed fan, what governs that change, more points on the motor, or simple change in voltage, and if so what governs that voltage change. I would just like to know more about what I am dealing with, so I have a good understanding of the measurments I get off tha there multi-meater thing-a-ma-giggy thing ayoump

THE motor is sealed unit..no rebuilt...the fan is very famous for collecting leaves and junk....I had a similar problem when the fan connector was toasted (melted down) look for it at the lower left side (looking at the fan) just to be sure....
The motor gets a signal from the coolant temperature sensor.........fan kicks in when AC is turned on.....$300 and some change plastic.....good luck!!